How Do You Prevent Thermal Runaway Propagation in a Multi-Cell Lithium Battery Pack?

Detailed Explanation

What is a battery protection circuit? and what is a BMS? both cover the electronics that stop a cell from ever reaching an unsafe state in the first place: overcharge, overdischarge, overcurrent, and (for a BMS) cell imbalance across a series string. All of that is first-fault prevention. Thermal runaway propagation is what happens after that first line of defence has already been defeated, whether by a design gap, a component failure, physical damage, or manufacturing defect, and one cell has genuinely entered runaway. At that point the question is no longer electrical. It's whether the pack's mechanical and thermal design can stop that one cell from taking its neighbours down with it.

Why Propagation Is a Different Failure Mode

A cell in thermal runaway is generating its own heat through an internal exothermic reaction, independent of any external circuit. Opening a MOSFET or blowing a fuse does not remove that heat source, because the reaction is happening inside the cell's own chemistry, not in the circuit around it. The protection circuit's job ends the moment the first cell crosses into runaway; everything that happens next, whether it stays contained to that one cell or spreads through the pack, is decided by the pack's mechanical and thermal construction rather than its electronics.

Heat Conduction Between Adjacent Cells

A cell in runaway can reach several hundred degrees Celsius at its casing. In a densely packed arrangement (cells touching or separated only by a thin structural spacer), that heat conducts directly into the neighbouring cells' casings. Lithium-ion cells have well-documented thermal thresholds above which their own internal separator and electrolyte begin to break down, so a hot neighbour can push an otherwise healthy cell into its own runaway purely through conducted heat, with no electrical fault involved at all.

Flammable Vent Gas and Enclosure Pressure

Cells in runaway vent gas: a mixture of flammable hydrocarbons, hydrogen, and other combustion products released as internal pressure builds beyond the cell's safety vent threshold. In a tightly packed, sealed enclosure, this hot, pressurised, often flammable gas has nowhere to go except into the space around the neighbouring cells. It can ignite on contact with the failing cell's exposed vent, and even without ignition, the heat carried by the gas itself is a second propagation path that is independent of direct cell-to-cell contact. A sealed pack with no engineered vent path effectively traps this gas against its neighbours, which is why an enclosure with no vent design tends to make a propagation event worse, not better, compared with an open or vented equivalent.

Mechanical and Thermal Design Mitigations

Cell Spacing and Airgaps

Physical distance between cells reduces conducted heat transfer, and an airgap (rather than direct cell-to-cell contact) slows that transfer further, since air is a comparatively poor thermal conductor next to solid metal or plastic cell casings. Spacing alone does not stop a determined propagation event on its own, particularly the vent-gas path described above, but it is a real and commonly used first layer of defence, and it is also one of the easiest mitigations to compromise for the sake of a smaller enclosure. Treat spacing decisions as something to validate against actual propagation test data for the specific cell and pack construction, not as a dimension chosen purely to fit a target volume.

Fire-Barrier and Intumescent Materials

Between cells, or between rows or modules of cells, many pack designs use a fire-resistant or intumescant barrier material: a material class that either resists heat transfer directly or expands and chars when exposed to heat, forming an insulating layer that slows conduction to the next cell. These materials come in mica-based sheets, ceramic-fibre composites, and intumescent coatings or pads, among other forms, and the right choice depends on the pack's thermal budget, available space, weight tolerance, and the specific propagation-test evidence the product needs to satisfy. This is a genuine class of design solution rather than a single named product, and material selection and thickness should be justified against the pack's actual worst-case single-cell heat release, not chosen by habit from a previous design.

Thermally Conductive, Electrically Isolating Interfaces

Where a design needs to actively pull heat away from cells (rather than simply block it from spreading), thermally conductive but electrically isolating gap-fillers, pads, or potting compounds sit between cells or between cells and a metal heat-spreading structure. These materials conduct heat efficiently while maintaining electrical isolation between cells that are at different potentials in a series string, which matters because a barrier or filler material that is also electrically conductive risks creating a short between cells instead of stopping one. Confirm both the thermal and dielectric-withstand specification of any such material against the pack's actual cell-to-cell voltage difference before selecting it.

Per-Cell Fusing as a Complementary Electrical Measure

Per-cell or per-string fusing, covered in detail for parallel-group current sharing in how do you design a series-parallel Li-ion battery pack configuration?, addresses a related but distinct failure: healthy cells in the same parallel group dumping current into a shorted neighbour. That electrical isolation reduces how much additional electrical energy a failing cell receives from its neighbours, which can reduce the severity of a runaway event, but it does nothing about the heat and vent gas the failing cell generates from its own stored chemical energy. Fusing is a genuinely useful complementary layer; it is not a substitute for the mechanical and thermal design covered in this article, and treating it as one is one of the more common gaps in pack safety design.

Vent Gas Routing and Enclosure Design

Because vent gas is both hot and potentially flammable, where it goes matters as much as how much of it there is. Good pack designs typically route venting away from adjacent cells and toward a dedicated exhaust path out of the enclosure, rather than allowing gas to accumulate in the space between cells or build pressure inside a sealed housing. Practical elements of this include directional vent features on the cell or module itself, pressure-relief provisions in the enclosure sized for the worst-case single-cell vent event, and physical channels that carry gas away from neighbouring cells rather than across them.

A fully sealed enclosure with no engineered vent path is a common design mistake made for water or dust ingress protection reasons without considering what happens during a runaway event: gas pressure builds inside the sealed housing until the enclosure itself fails, at which point the released gas and heat are no longer contained at all, and any positive spacing or barrier work done between cells is undermined by an enclosure that ruptures uncontrollably rather than venting in a designed, directed way. Ingress protection and propagation venting need to be reconciled as a single design requirement, not treated as if one automatically accommodates the other.

The BMS and Protection Circuit's Role Stops at the First Fault

It is worth restating plainly because it is the most common point of confusion on this topic: the protection circuit and BMS described in what is a battery protection circuit? and what is a BMS? are the correct and necessary tools for preventing the first cell fault, whether that is overcharge, overdischarge, overcurrent, or a series-string imbalance. Propagation-prevention design is what a pack needs for the scenario where that first line of defence has already been bypassed or has failed, whether through a design gap, a damaged cell, a manufacturing defect, or physical abuse in the field. A pack with an excellent BMS and no mechanical propagation design is still exposed to a cascading failure if any single cell reaches runaway by a path the BMS could not see or stop; a pack with strong propagation-prevention design and a poor BMS will simply reach that first fault more often. Both layers are required, and neither substitutes for the other.

Relevant Standards

UL 1973 sets safety requirements for batteries used in stationary, vehicle auxiliary power, and light electric rail applications, and its scope includes construction and test requirements relevant to a pack's tolerance of internal cell faults. IEC 62133-2 sets safety requirements and single-fault tests (overcharge, external short, crush, and similar) for portable sealed secondary lithium cells and batteries, and is commonly required for consumer product certification. UL 9540A is a dedicated test method for evaluating whether thermal runaway in one cell propagates to neighbouring cells, modules, or units in a battery energy storage system, and its cell-to-cell and module-level test protocols are the closest fit in this list to propagation testing specifically, though it was developed primarily for stationary energy storage systems rather than portable consumer packs.

None of these three standards is a complete, self-contained checklist for propagation safety in every product category: which standard (or combination) applies, and which specific clauses are relevant, depends on the product's application, jurisdiction, and energy content. Standards editions and scopes change; confirm the current edition and its applicability to your specific product category with the standards body or a certification body before treating any of this as a fixed requirement list, and do not assume a standard written for one application category (stationary storage, portable consumer devices, or transport) automatically covers propagation risk in a different one.

Common Mistakes

• Relying on the BMS or per-cell fusing alone and assuming it addresses propagation. Both are genuinely necessary, but neither removes the heat and vent gas a cell already in runaway generates from its own stored energy; propagation resistance is a separate mechanical and thermal design question, not a side effect of good electrical protection.
• Reducing cell spacing to save enclosure volume without re-validating propagation risk. Spacing decisions made purely against a volume target, rather than against actual thermal and vent-gas behaviour for the specific cell and pack construction, are one of the most common places propagation-prevention design quietly erodes during a product's development.
• Sealing the enclosure for ingress protection with no engineered vent path for runaway gas. A sealed housing that has no dedicated pressure-relief or venting provision can turn a single-cell event into an uncontrolled enclosure rupture instead of a designed, directed release, which typically makes the outcome worse rather than better.
• Treating a passed UN 38.3 test as equivalent to propagation safety. UN 38.3 qualifies a cell or pack against transport stresses, not in-service propagation between cells; the two address different hazards, and passing one says nothing about the other.
• Choosing a barrier or gap-fill material for its thermal performance alone without checking its dielectric withstand. A thermally effective material that is also electrically conductive enough to bridge cells at different potentials in a series string trades one failure mode for another.

Decision Criteria: When Propagation-Prevention Design Effort Is Proportionate

The amount of engineering effort that is proportionate here scales with a handful of factors, and there is no single threshold that applies to every product:

• Cell count and pack energy content. A single-cell product has no propagation to prevent, since there is no neighbouring cell to affect; the moment a design moves to multiple cells, and especially as cell count and total pack energy rise, the consequence of a single cell's runaway spreading grows accordingly.
• Proximity to people during normal use. A pack worn on the body, held in the hand, or installed close to occupied space warrants a higher bar than one in an unattended enclosure in a low-traffic location, simply because the consequences of a failure differ.
• Energy density and chemistry. Higher energy density chemistries generally carry more stored energy per cell available to release during a runaway event, which raises the stakes of a propagation failure relative to a lower energy density chemistry of the same physical size, though the specific chemistry's thermal stability characteristics also matter and should be confirmed against the manufacturer's own safety data rather than assumed from the chemistry family alone.
• Regulatory and market requirements. Some jurisdictions and product categories require specific propagation-relevant testing or certification as a condition of sale; confirm the applicable requirement for your specific market and product category early, since retrofitting mechanical propagation mitigations into a design that was not built with them in mind is typically far more disruptive than accounting for them from the start.

If you are designing a multi-cell pack for a regulated market, or one where a single-cell failure has real consequences for people or property, it is worth getting the propagation-safety design reviewed against the applicable standards early, before the mechanical construction and enclosure are finalised. Zeus Design's engineering team works through this kind of pack-level mechanical, thermal, and certification planning alongside the electrical design for IoT, industrial, and mobility products.